1. Field of the Invention
The present invention relates to a wireless local area network (WLAN) system, and more particularly, to a method and apparatus for transmitting a physical layer convergence procedure (PLCP) protocol data unit (PPDU).
2. Related Art
With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted.
In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.
An IEEE 802.11n HT WLAN system employs an HT green field physical layer convergence procedure (PLCP) protocol data unit (PPDU) format which is a PPDU format designed effectively for an HT station (STA) and which can be used in a system consisting of only HT STAs supporting IEEE 802.11n in addition to a PPDU format supporting a legacy STA. In addition, an HT-mixed PPDU format which is a PPDU format defined such that a system in which the legacy STA and the HT STA coexist can support an HT system.
With the widespread use of a wireless local area network (WLAN) and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. A next generation WLAN system supporting a very high throughput (VHT) is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of 1 Gbps or higher in a medium access control (MAC) service access point (SAP).
The next generation WLAN system allows simultaneous channel access of a plurality of VHT-STAs for the effective use of a radio channel. For this, a multi-user multiple input multiple output (MU-MIMO)-based transmission using multiple antennas is supported. A VHT-access point (AP) can perform spatial division multiple access (SDMA) transmission for transmitting spatial-multiplexed data to the plurality of VHT-STAs. When data is simultaneously transmitted by distributing a plurality of spatial streams to the plurality of STAs by the use of a plurality of antennas, an overall throughput of the WLAN system can be increased.
When using the MU-MIMO transmission method for transmitting data simultaneously with respect to the plurality of VHT-STAs, a physical layer convergence procedure (PLCP) protocol data unit (PPDU) including data intended to be transmitted to the plurality of VHT-STAs paired with the VHT-AP is transmitted through orthogonal frequency division multiplexing (OFDM) symbols, and the number of OFDM symbols is identical for each VTH-STA. However, a size of data intended to be transmitted to each VHT-STA may differ. Therefore, there is a need for a method for generating and transmitting a PPDU in such a manner that a specific padding bit is appended according to a size of data transmitted to a VHT-STA so that PPDUs to be transmitted to a plurality of STAs have the same bit size and thus are transmitted through the same number of OFDM symbols.